Method of clay treatment



United States Patent Application June 10, '1949, Serial No. 98,410

9 Claims. (Cl. 23-110) No Drawing.

This invention relates to the treatment of clay and more particularly to a method of treating clay to improve its brightness. Brightness, as herein used, is a relative term based upon the reflective power of a freshly prepared surface of magnesium oxide which is ordinarily taken as having 100% brightness.

Many of the commercial uses to which clay may be applied, e. g., clay for paper coating, demands a high degree of brightness. Because of the demand for brightness in clay for the paper industry, as well as for the ceramics and other clay consuming industries, many large deposits which might otherwise be accessible have not been developed.

It has been found that many clays which are suitable from the standpoint of physical characteristics other than brightness are so lacking in brightness as to be unsuitable for mose commercial purposes.

Moreover, present methods of refinement, including present methods of bleaching, have not servedtoovercome this lack of brightness inherent in many clay deposits. 1

Brightness requirements must be considered in connection with color requirements. The practice of the orthodox bleaching processes may resultin imparting a gray tone to the clay which renders it unsatisfactory. Clay in this condition is said to be overbleached. It may be said that in general a clay on the buff sidefis'fmore desirable than a clay on the gray" side evenif itsf bright ness is somewhat lower. In consequence, the-limit of brightness improvement in the treatment of clay, notably coating clay, has been the limit of brightness obtainable without changing such color as remains in the clay from bufi to gray. It will be understood that although the amount of color difference present in coating clays generally is relatively small, it is usually observable by a side-to-side comparison of different specimens. In coat-L ing clays, a brightness difference of as much as three percentage points, e. g., from 85% to 88% or from 88% to 90%, is of very great technicaland economic importance. v

Representative present bleaching practice consists in treating a degritted crude clay slip from which an additional coarser particle fraction amounting to about 20% to 30% of the degritted crude clay weight has preliminarily been removed as a substantially unbleachable component, according to prior art, with "afreducin'g agent such as zinc or sodium-hydrosulphite together with an acidic material such as aluminum sulphate or sulphuric acid. By the term hydrosulphite I refer to thesalts of hydrosulphurous acid, HzSaOe. The acid condition estab; lished in the slip is maintained at about pH 4. Care'is' taken to avoid the use of'an excess of hydrostflphite and, in general, the practice has been to use 4 to' 6 lbs. of S02 as hydrosulphite to the ton of clay (weights on the dry basis) when bleaching secondary clays 's'uch Georgia Fall-line clays. If larger amounts of hydroi sulphite than this are used under theseprior art condi 2,758,010 Patented Aug. 7, 19 56 2 ondary Georgia Fall-line clays the gray coloration develops. In fact, in order to avoid the formation of gray coloration, it has been preferred practice to use insufiicient hydrosulphite even for a complete reducing action if necessary, and thus to avoid the gray coloration and leave a residual bufi color which is preferable to the gray for most commercial uses.

I 'have made the surprising discovery that a clay slip which has been heavily bleached to a pronounced grayness contains two fractions, one of which is markedly gray and'ofpoor color and the other of which is relatively free from grayand poor color and hence of an improved order ofbrightn'essf'and that these fractions may be separated so as to produce from a given crude clay a product which is markedly superior to that obtainable from the crude by present methods.

I preferably employ hydrosulphite additions Within the range which was formerly believed to beincompatible with the production of high quality clay. For secondary clays such as those found along the Georgia Fall-line, I have used hydrosulphite additions of 10 to 30 lbs. of

- active'SOzper ton of dry clay as distinguished from the sulphite were considered dangerously large.

I preferably bleach when the slip is maintained at a pH in the range 2.5-3.2, for a period of at least 10 minutes' after bleaching action begins. This acidity may be brought about by the addition of an acid or acidic material; mineral acids such as sulphuric are generally less prior-art where additions of 8 lbs. of active S02 as hydro-' costly and to be preferred. Although subsequent ph'ysi- "additions appears to be in the greater reduction in buff tions of acidity, etc., on clays such as, for example, seccolor which results. To remove the gray component coincidentally. formed, 1 first redisperse the clay after the above overbleaching treatment by adding an alkaline agent such as ammonia or other suitable alkali to readjust the pH of the slip to about pH 5.5 to pH,-'8.5

(preferably to neutral or slightly alkaline), in the pres-,

ence of a small amount of dispersing agent such as a few tenths of a per cent ofa polyphosphate, sodium hexametaphosphate, etc. Dispersion is, of course, hastened byxvigorous mechanical agitation of themixture. When the clay has been -redispersed, a strongly discolored sedi ment may be'separated by allowing the'mixture to stand quietly until .a gravity'separation is effected; or by sub-' jecting'the clay slip to acentrifuging action. When the sediment is' separated by gravity settling, the supernatant clay is poured ofif, coagulated, filtered and dried.

The dried clay resulting from this treatment is found to havea brightness 3% to 4% higher than the same type of clay as bleached by prior art. in brightness is very important in thepaper coating field where brightness is always at a premium.

Another and very material advantage in my invention arises from the fact that, whereas the prior art teaches the separation of the coarser particle size fraction from the crude clay after degritting but before bleaching, my process may be applied to the entire degritted clay including coarse particles. By the prior practice, about 30% of the degritted clay is discarded as coarse particles to which prior bleaching practice was unsuitable, whereas by my process not more than 10% and usually about 5% This differenceor less of the entire weight of the degritted clay is discarded as the discolored fraction, leaving the remaining 80% or more of the clay as well bleached and of good brightness. These yields naturally are somewhat dependent upon the quality of the crude clay.

EXAMPLE 1 A quantity of crude secondary clay from the Georgia Fall-line was mixed with water to make a conveniently handleable slip (about 25% slip in clays of this type). The slip was subjected to a stirring and shearing action in the presence of 0.3% polyphosphate as a dispersing agent. The dispersed clay was then allowed to stand quietly to permit degritting by gravity settling. All of these steps are well known in the prior art.

I next diluted the degritted slip to form a 15% slip and added 0.65% active S02 as dry zinc hydrosulphite based on the dry weight of the clay. Sufficient sulphuric acid was added to bring the pH to about pH 2.5 to pH 3.2. The slip was stirred gently to mix the hydrosulphite and acid with the slip and allowed to stand about 30 minutes. At the end of this time the slip was neutralized with ammonia and about 0.5% of polyphosphate added to act as a dispersing agent.

After the dispersion of the slip was effected, it was allowed to settle and a fraction separated by sedimentation. The supernatant suspension was separated from the discolored sediment, coagulated, filtered and dried.

A control slip of the same clay was subjected to the bleaching treatment of the prior art, that is to say, it was degritted, coarse particles removed in an amount of about 30% of the total weight, inasmuch as by prior art this component is considered as relatively unbleachable and is removed prior to bleaching, and the remaining slip brought to a pH of about, pH 3 to pH 4 and about 0.25% of active S02 as hydrosulphite added. The resulting product was filtered and retained.

The two samples so treated were dried and the brightness was determined by a General Electric Reflection meter using a wave length of about 457 millimicrons with the results shown in Table A.

Table A Bleached Discard of Clay Re- Brlghtness, Unbleachcovery on Percent able Clay, Degrittcd Percent Slip,

Percent Slip treated by my invention"... 87. s 2. 5 91. 5 Slip bleached by prior art process. 83. 3 30.0 70. .Dlfierence in brightness 4.

EXAMPLE 2 Table B Brightness Brightness after treat- Brightness after prior ment accofd- Improveart bleach ing to present merit invention 4 EXAMPLE 3 I have also taken a crude secondary clay of quality below that considered standard for bleaching clay according to the prior art, and by applying my process arrived at a bleached clay result well above that produced by prior art processes.

A degritted clay slurry of this quality was treated with 1.0% of active S02 as zinc hydrosulphite, and sufficient sulphuric acid to establish an initial pH of 2.8. This pH of 2.8 was maintained by subsequent small additions of sulphuric acid over a period of 15 minutes subsequent to the start of the bleaching reaction. These latter additions of acid were made to restore the condition of acidity as the bleaching reaction tends to reduce the acidity as it progresses. The reaction was then allowed to go to completion by standing without further additions overnight. The bleached clay was neutralized and dispersed and the gray, discolored component removed as previously described. The bleaching results obtained on this clay as compared with bleaching results obtained by the prior art process are illustrated in Table C. Brightness measurements on the removed gray discolored component after filtering and drying gave a brightness of 80%.

Table C Brightness Brightness after treat- Brightness alter' prior mcnt by Improveart bleach process of ment present inventlon Similar improvement in brightness may be achieved on clays which according to the prior art are considered of good quality for bleaching.

EXAMPLE 4 A ceramic clay of the secondary Georgia type was treated as were the clays of Example 1. Here again the improvement in brightness in the fired ceramic clay was sufiicient potentially to put the secondary ceramic in a new technical and market position as indicated in Table D.

In other instances I have found that certain poor colored clays which the prior art treatment is unable to bring to the required level of brightness have been made commercially acceptable in brightness by my treatment.

My researches have shown that the discolored sediment which results from my treatment is not identical with the coarser particles of clay which would ordinarily settle on long standing in clays not treated by my process. I believe that the discrimination depends upon the reaction of the excess hydrosulphite in acid solution of pH 2.5 to 3.2 with the fixed or insolubilizable iron which probably exists in association with other extraneous minerals or even with clay particles as a surface film of hydr'ous ferric oxide which is known to exist in very stable colloidal suspension. It is probable that several forms of reduced sulphur compounds are produced by the reaction of the hydrosulphites in acid solution and that the'se'subs'tances then react with the iron surfaces to of suspension. Although this theory is consistent with the observed facts, and with the theories of colloidal chemistry, I do not bind myself to this theory.

It will be understood that while I have illustrated and described the presently preferred application of my invention, it may be otherwise embodied or practiced within the scope of the following claims.

I claim:

1. A method of treating clay comprising the steps of forming an aqueous slurry of the clay to be treated, bleaching the slurry with a hydrosulphite bleaching agent until the slurry contains two fractions in suspension one of which is predominately gray in color, all the while maintaining the pH below 3.2, adjusting the pH to the range 5.5 to 8.5 and separating the predominantly gray fraction from the slurry by sedimentation.

2. A method of treating clay comprising the steps of forming an aqueous slurry of the clay to be treated, adding sufficient Zinc hydrosulphite to produce about 1.0% S02 adjusting the pH of the slurry to about 2.8 with sulfuric acid, maintaining the pH of the slurry to within the range 2.5 to 3.2 by additions of sulfuric acid while, bleaching the slurry with a hydrosulphite bleaching agent until the slurry contains two fractions in suspension one of which is predominately gray, adjusting the pH to the range 5.5 to 8.5 and separating the gray fraction from the slurry by sedimentation.

3. A method of treating clay or the like comprising the steps of forming an aqueous slurry of the clay to be treated, adjusting the pH of the resulting slurry to about pH 2.5 to pH 3.2, adding to the slurry a solution of hydrosulphite whose dry weight content of S02 is in excess of 0.4% of the dry Weight of the clay to form two fractions in suspension one of which is predominately gray, redispersing the resulting mixture after adjusting its pH to about pH 5.5 to pH 8.5, and separating the resulting gray fraction by sedimentation.

4. A method of treating clay or the like to improve its brightness comprising the steps of forming an aqueous slurry of the clay to be treated, adjusting the pH of the resulting slurry to about pH 2.5 to pH 3.2, adding to the slurry a solution of active S02 as hydrosulphite whose dry weight content of S02 is about 0.4 to 1.5% of the dry weight of the clay to form two fractions in suspension one of which is predominately gray, redispersing the resulting mixture after adjusting its pH to about pH 5.5 to pH 8.5 and separating the resulting gray fraction by sedimentation.

5. A method of treating clay or the like to improve its brightness comprising the steps of forming an aqueous slurry of the clay to be treated, adjusting the pH of the resulting slurry to about pH 2.8, adding to the slurry a solution of active S02 as hydrosulphite whose dry weight content of S02 is about 0.4 to 1.5 of the dry weight of the clay to form two fractions in suspension one of which is predominately gray in color, redispersing the resulting mixture after adjusting its pH to about pH 5.5 to pH 8.5 and separating the resulting gray frac tion by sedimentation.

6. A method of treating clay or the like to improve its brightness comprising the steps of forming an aqueous slurry of the clay to be treated, adjusting the pH of the resulting slurry to about pH 2.5 to pH 3.2, adding to the slurry a solution of active S02 as zinc hydrosulphite whose dry weight content of S02 is about 0.4 to 1.5 of the dry weight of the clay to form two fractions in suspension one of which is predominately gray in color, redispersing resulting mixture after adjusting its pH to about pH 5.5 to pH 8.5 and separating the resulting gray fraction by sedimentation.

7. A method of treating clay or the like to improve its brightness comprising the steps of forming an aqueous slurry of the clay to be treated, adjusting the pH of the resulting slurry to about pH 2.5 to pH 3.2, adding to the slurry a solution of active S02 as sodium hydrosulphite whose dry weight content of S02 is about 0.4% to 1.5% of the dry Weight of the clay to form two fractions in suspension one of which is predominately gray in color, redispersing the resulting mixture after adjusting its pH to about pH 5.5 to pH 8.5, and separating the resulting gray fraction by sedimentation.

8. A method of treating clay or the like to improve its brightness comprising the steps of forming an aqueous slurry of the clay to be treated, adjusting the pH of the resulting slurry to about pH 2.5 to pH 3.2 by the addition of a mineral acid, adding to the slurry a solution of active S02 as hydrosulphite whose dry weight content of S02 is about 0.4% to 1.5 of the dry weight of the clay to form two fractions in suspension one of which is predominately gray in color, redispersing the resulting mixture after adjusting its pH to about pH 5.5 to pH 8.5, and separating the resulting gray fraction by sedimentation.

9. A method of treating clay or the like to improve its brightness comprising the steps of forming an aqueous slurry of the clay to be treated, adjusting the pH of the resulting slurry to about pH 2.5 to pH 3.2 by the addition of a sulphuric acid, adding to the slurry a solution of active S02 as hydrosulphite whose dry weight content of S02 is about 0.4% to 1.5% of the dry weight of the clay to form two fractions in suspension one of which is predominately gray in color, redispersing the resulting mixture after adjusting its pH to about pH 5.5 to pH 8.5, and separating the resulting gray fraction by sedimentation.

References Cited in the file of this patent UNITED STATES PATENTS 2,020,132 Binns Nov. 5, 1935 2,147,774 Lyons Feb. 21, 1939 2,149,506 Bump Mar. 7, 1939 2,251,256 Feldenheimer July 29, 1941 2,339,594 Williams Jan. 18, 1944 

1. A METHOD OF TREATING CLAY COMPRISING THE STEPS OF FORMING AN AQUEOUS SLURRY OF THE CLAY TO BE TREATED, BLEACHING THE SLURRY WITH A HYDROSULPHITE BLEACHING AGENT UNTIL THE SLURRY CONTAINS TWO FRACTIONS IN SUSPENSION ONE OF WHICH IS PREDOMINATELY GRAY IN COLOR, ALL THE WHILE MAINTAINING THE PH BELOW 3.2, ADJUSTING THE PH TO THE RANGE 5.5 TO 8.5 AND SEPARATING THE PREDOMINANTLY GRAY FRACTION FROM THE SLURRY BY SEDIMENTATION. 